[Amps] Amps] water purity/conductivity in water cooled tube, > amplifiers

[John Lyles]

Steve,
You have 3600 VDC and want to keep the leakage current small, in the uA, 
no mA. So if that is, say 100 uA, then R=3600/100e-6, or 36 Megohms.

RCA recommended never lower than 10 Megohm path resistance to prevent 
electrolysis in the water connections.

I use PEX tubing, used for home water pipes now, and it can be formed 
into a coil by heating it carefully with one or two heat guns and 
winding it around a wood dowel. Then it will retain it's form when 
cooled off. If you screw up, just reheat and form it again. Amazing 
stuff. I am not sure if it is available in 3/8 inch. Eimac recommends 
against silicone tubing in their literature, although I am using red 
silicone-based radiator hoses for megawatt amplifiers that I build for 
work.

You have three variables, the ID of the insulated piping, the length 
from HV to ground through it and the resistivity of the water. I use 
resistivity not conductivity since the value is easy to understand and 
the dimensional units cancel out correctly in the formula. The pipe is a 
resistor of high value depending on what these variables are.

With 3/8 tubing chosen, I will assume 0.375 inch ID for this example, 
but you can check the actual ID and correct if I guessed wrong.
Convert it to SI units, 0.95 cm. Also, pick a length of your pipes that 
is convenient to fit in the cabinet, remembering that the hose must be 
floated on insulator or an insulated form for most of the length. I 
started with 12 inches, or 45.97 cm long per hose. 1 Megohm-cm water 
chosen for this example. This is so short that coiling might not be 
needed depending on the cabinet space.

Since the ID is 0.95 cm, area is 0.709 cm^2.
R = resistivity x (L/A)
L is 45.97 cm
A is 0.709 cm^2
resistivity is 1 Megohm-cm
R= 64 Megohms

This is twice what we wanted, so you can use 500 Kohm-com water with 
this length of tubing and diameter. If you double the length, the 
current goes in half, etc. I would recommend not going to below 500 
kohm-cm water though.

Remember that this is per hose, so with two hoses (in and out) the total 
current loading on the power supply from the water pipes is twice what 
was calculated above. This is what a DC hipotter should measure if the 
tube is off, no filament power applied.

Also, temp rise is dependent on flow and dissipation.
P (kW) = 0.2648 x (Tout-Tin) x Flow (GPM) for water
You don't want to create a very high linear water flow rate through the 
tube and piping that is beyond acceptable practice as it is noisy and 
can cause erosion as well as turbulent conditions in the anode. So that 
weighs into your 3/8 tubing decision.

73
John
K5PRO

> Date: Fri, 13 Apr 2018 21:40:20 -0400
> From: Steve Bookout <steve at nr4m.com>
> Hello all,
...
> My general plan was to use 3/8 inch silicone tubing, to and from, the
> tube boiler.? In order to give myself some 'dielectric length', I was
> going to wrap several turns of the tubing around a round form in a
> single layer.? This would be kind of like coaxial choke some wrap on a
> piece of PVC.? ??? I think I would rather have 3 or 4 feet of 'series
> water', than have the HV only 10 inches of water from conductive 'stuff'.
>
> Plan on putting a micro ammeter from a metal water fitting? to chassis
> in order to measure the conductivity from the water to chassis, at some
> point in the water circuit.
>
> I know that really pure water is actually corrosive and will do it's
> best to gain ions in order to get to some natural level of
> minerals/contaminants.??? I built the coolers out of several pieces of
> brass and hard silver brazed it all together.? I know the electrical
> current will cause an etching or eroding effect of the materials in the
> cooler, but I used what I had.?? At this point, I have no idea how often
> I will changing? out the water.? Could be after only 10 hours of use, or
> it may be 100 hours and the cooler may rot out in no time.
>
> So, I need to know what the 'big boys' do in industry.? At what point,
> in uS/cm, do I change out the water? The metering of the water
> conductivity would be measure 3 or 4 feet from the tube boiler (@ 3600
> volts)? How many inches (feet) of 3/8 inch dia distilled/low ion water,
> in silicon tubing, do I need to have between 3600 volts and gnd (thru a
> meter?)

...
> 73 de Steve, NR4M
> ------------------------------
>
> Message: 4
> Date: Fri, 13 Apr 2018 21:43:33 -0400
> From: Steve Bookout <steve at nr4m.com>
> Subject: [Amps] more on water cooling an amp
> I just realized that I could use my Hi-pot tester to test things and
> help set it up.